1. Field of the Invention
The present invention relates to a method and related communication device in a wireless communication system for improving a hybrid automatic repeat request (HARQ) procedure, and more particularly, to a method and related communication device for improving an uplink HARQ procedure in the transition between transmission time interval (TTI) bundling and non-bundling.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The radio protocol stacks of the E-UTRAN is given including a radio resource control layer (RRC), a packet data convergence protocol layer (PDCP), a radio link control layer (RLC), a medium access control layer (MAC), and a physical layer (PHY).
In order to improve uplink (UL) coverage of LTE at cell edge, transmission time interval (TTI) bundling is introduced for frequency division duplex (FDD). Activation or deactivation of TTI bundling is controlled by a radio resource control (RRC) command and a parameter TTI_BUNDLE_SIZE, which provides the number of TTIs of a TTI bundle, is 4. Within a TTI bundle, hybrid automatic repeat request (HARQ) retransmissions are non-adaptive and are performed without waiting for feedback of previous transmissions. A HARQ feedback, e.g. positive-acknowledgement (ACK)/negative-acknowledgement (NACK) information, for a TTI bundle is only received for the TTI corresponding to the TTI_BUNDLE_SIZE, which is the last TTI of the bundle. A retransmission of a TTI bundle is also a TTI bundle. In addition, the number of HARQ processes is decreased from 8 to 4 and the UL HARQ round trip time (RTT) after TTI bundling activation is increased from 8 to 16.
A HARQ entity at a UE maintains a number of parallel HARQ processes allowing transmissions to take place continuously while waiting for the HARQ feedback on successful or unsuccessful receptions of previous transmissions, and also carries new transmission parameters, e.g. a new data indicator (NDI) and spectral resources including a physical resource block (PRB) and a modulation and coding scheme (MCS), for each HARQ process. At a given TTI, if an UL grant is indicated for the TTI, the HARQ entity identifies a HARQ process where a transmission in the TTI should take place. Based on a physical downlink control channel (PDCCH), the HARQ entity also determines whether a retransmission is adaptive or non-adaptive and provides transmission parameters for non-adaptive retransmission. When an ACK for a HARQ process is received, the HARQ entity considers the HARQ process suspended and stops generating non-adaptive retransmission for the HARQ process.
Please refer to FIG. 1, which is a timing diagram illustrating HARQ processes when TTI bundling is activated according to the prior art. In FIG. 1, a UE receives a NACK for a bundle of HARQ process 0 from the eNB at the subframe (n+4) when the last TTI of the bundle of HARQ process 0 is the subframe n, and when the first TTI of the bundle of HARQ process 0 is the beginning of an RTT, the UE does not perform a retransmission of the HARQ process 0 until a next RTT.
However, so far the 3GPP specification has not captured the HARQ procedure for unfinished HARQ processes after TTI bundling is activated or deactivated. Therefore, the UE may have no idea for whether to continue unfinished HARQ processes after TTI bundling is activated or deactivated.